Object.defineProperties and Object.defineProperty overwrite existing values

[raskad]

Describe the bug
Object.defineProperties and Object.defineProperty overwrite existing values when they only should set attributes.

To Reproduce
This JavaScript code reproduces the issue:

var arr = [];
Object.defineProperties(arr, {
  length: {
    writable: false
  }
});
arr.length

Expected behavior
In the example arr.length is overwritten with undefined, but the Object.defineProperties call should only affect the writable attribute while leaving the value of the property unchanged.

Build environment:

• OS: [Linux]
• Version: [master]
• Target triple: [x86_64-unknown-linux-gnu]
• Rustc version: [rustc 1.52.0 (88f19c6da 2021-05-03)]